This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 to Korean Patent Applications No. 98-15991 filed May 4, 1998, and No. 98-60009 filed Dec. 29, 1998; the entire contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a proton exchange membrane (PEM) fuel cell, and more particularly, to a method for preparing a slurry for forming a catalyst layer of a PEM fuel cell electrode having an improved processing stability and power output characteristics, a method for fabricating membrane/electrode assembly (MEA) for the PEM fuel cell and a PEM fuel cell produced by the MEA fabricating method.
2. Description of the Related Art
A proton exchange membrane (PEM) fuel cell is a potential clean energy source which can replace fossil fuels and has a high current density and energy conversion capability. Also, the PEM fuel cell is operable at room temperature and can be miniaturized and hermetically fabricated, and thus are widely applicable in such fields as pollution-free automobile industry, home-use power generation systems, mobile communications equipments, medical devices, military equipments, aerospace equipments and the like.
A PEM fuel cell is a power generation system for producing direct current electricity by an electrochemical reaction between hydrogen and oxygen, and the basic structure thereof is shown in FIG. 1. In FIG. 1, a general PEM fuel cell is constructed such that a proton exchange membrane 13 is interposed between an anode 11 and a cathode 12. A PEM 13 is 50 to 200 xcexcm thick and is formed of a solid polymer electrolyte. In the PEM fuel cell, the anode 11 and the cathode 12 both have a backing layer (not shown) for supplying fuel gases and a catalyst layer where oxidation/reduction reaction of gaseous fuels takes place.
The oxidation/reduction reactions taking place at the PEM fuel cell are represented by the Equations (1) and (2).
That is to say, the oxidation reaction as represented by the Equation (1) takes place at the anode 11 of the gas diffusion electrode so that hydrogen molecules are converted into protons and electrons. The protons are transferred to the cathode 12 via the PEM 13. At the cathode 12, the reduction reaction as represented by the Equation (2) takes place, so that oxygen molecules receive electrons to be converted into oxygen ions, which are then reacted with protons produced from the anode 11 to then be converted into water molecules.
2H2xe2x86x924H++4exe2x88x92xe2x80x83xe2x80x83[Equation 1]
O2+4exe2x88x92xe2x86x922O2xe2x88x922O2xe2x88x92+4H+xe2x86x922H2Oxe2x80x83xe2x80x83[Equation 2]
The catalyst layer is formed on a backing layer in the gas diffusion electrode of the PEM fuel cell. The backing layer is formed of a carbon cloth or a carbon paper and its surface is treated with polytetrafluorethylene (PTFE) so that reactant gases and water transferred to the PEM and generated from the above reaction can easily penetrate therethrough.
The catalyst layer is typically formed of platinized carbon powder (Pt/C). Here, carbon serves to extend the reaction site of introduced fuels, and platinum acts as a catalyst in the oxidation/reduction reaction of the gaseous reactants, that is, hydrogen and oxygen.
Since the PEM fuel cell uses a solid polymer as an electrolyte, the boundary between the electrode and the electrolyte is two-dimensional, which reduces the catalyst utilizing efficiency, compared to a liquid electrolyte. Thus, it is necessary to make a three-dimensional boundary between the electrode and the electrolyte.
Conventionally, in order to prepare a catalyst layer of the gas diffusion electrode, powdered Pt/C is used as a main component and PTFE is used as a binder. Here, since the oxidation/reduction reaction of a gaseous fuel in the presence of a catalyst occurs only at the boundary between the electrode and the electrolyte, the catalyst utilizing efficiency is very low. Thus, to obtain a current density of a practical level, a catalytic loading amount must be increased to about 4 mg/cm2. However, in this case, due to high costs, the thus-formed electrode is used only for special purposes.
To overcome the above problems, as described in U.S. Pat. No. 4,876,115, for the purpose of limiting catalyst loadings to less than 0.50 mg/cm2, a solution containing a proton conducting material selected from perfluorocarbon polymer commercially available from E.I. DuPont under the trademark Nafion(copyright) and ruthenium dioxide is coated on the electrode once or twice to form a single or double layer formed of the proton conducting material. However, in the PTFE-bonded electrode mainly for use in a phosphoric acid type fuel cells, a large amount of a binder such as PTFE, e.g., over 30 wt %, must be used in order to prevent phosphoric acid used as a liquid electrolyte from penerating into the electrode. Thus, Pt in the catalyst layer may be covered by PTFE, which lowers the catalyst utilizing efficiency. Also, since a sufficient catalyst loading cannot be attained by performing coating only once, the coating step must be repeated twice as described above.
Also, U.S. Pat. No. 5,234,777 discloses a method for preparing a proton exchange membrane comprising the steps of preparing an ink-form of a mixture containing Na+ form-PFSI polymer by mixing Pt/C and perfluorosulfonate ionomer (PFSI) and adding sodium hydroxide solution thereto, forming a thin film by directly coating the mixture on the surface of a solid polymer electrolyte or on a plate-shaped releasable substrate, transferring the thin film to the surface of the solid polymer electrolyte by a hot pressing method and curing the same. However, this method requires a pre-treatment process for converting the solid polymer electrolyte as the PEM into a Na+ form before performing the coating step. Also, a protonating step for converting a Na+-form PFSI electrode/Na+-form solid polymer electrolyte membrane composite film into a protonated PFSI must be performed after performing the coating and curing steps. Therefore, the process is complicated and the processing time is long.
The Nafion solution commercially available from E. I. DuPont contains a large amount of alcohol having low specific gravity as well as a perfluorocarbon polymer as a main component and analogs thereof. Thus, in the course of fabricating a PEM fuel cell, a large amount of an alcohol solvent exists in the mixture for forming a catalyst layer having a predetermined Nafion polymer content, and easily penetrates into an electrode substrate.
Therefore, a considerable amount of a Pt catalyst penetrates into an electrode support so that it cannot take part in an electrochemical reaction, thereby lowering the Pt catalyst utilizing efficiency. Also, the viscosity of the catalyst layer composition prepared by the conventional method is less than 100 cp (centipoise). It is difficult to maintain a predetermined viscosity during the step of coating the catalyst layer composition onto the electrode support. Also, it is not possible to adopt a continuous production system using a tape casting.
A technology suitable for resolving such problems has not been developed as of yet.
To solve the above problems, it is an objective of the present invention to provide a method for preparing a slurry for forming a catalyst layer for a proton exchange membrane (PEM) fuel cell electrode with no problem arising due to alcohol remaining in the slurry.
It is another objective of the present invention to provide a method for fabricating a PEM fuel cell having a Pt loading in an electrode higher than 0.2 mg/cm2 by performing coating only once and capable of easily forming a catalyst layer directly on an electrode support to thus simplify the preparation process thereof, by using the slurry for forming a catalyst layer for a PEM fuel cell.
It is still another objective of the present invention to provide a PEM fuel cell prepared by the method.
Accordingly, to achieve the first objective, there is provided a method for preparing a slurry for forming a catalyst layer of a proton exchange membrane (PEM) fuel cell comprising the steps of:
(a) adding an MOH solution to a perfluorosulfonate ionomer (PFSI) solution to convert PFSI in the PFSI solution into an M+ form-PFSI solution, wherein M is an alkaline metal selected from the group consisting of Li, Na and K;
(b) adding an organic polar solvent having a higher boiling point than that of alcohol remaining in the PFSI solution to a mixed solution obtained in step (a) and heating the mixture at a temperature range of the boiling point of the alcohol to 20xc2x0 C. higher than the boiling point to remove the remaining alcohol to obtain a pretreated PFSI solution; and
(c) mixing the pretreated PFSI solution with Pt/C to form a slurry for forming a catalyst layer of a PEM fuel cell.
To achieve the second objective, there is provided a method for fabricating a proton exchange membrane (PEM) fuel cell comprising the steps of:
(a) adding an MOH solution to a perfluorosulfonate ionomer (PFSI) solution to convert PFSI in the PFSI solution into an M+ form-PFSI solution, wherein M is an alkaline metal selected from the group consisting of Li, Na and K;
(b) adding an organic polar solvent having a higher boiling point than that of alcohol remaining in the PFSI solution to a mixed solution obtained in step (a) and heating the mixture at a temperature range of the boiling point of the alcohol to 20xc2x0 C. higher than the boiling point to remove the remaining alcohol to obtain a pretreated PFSI solution;
(c) mixing the pretreated PFSI solution with Pt/C to form a slurry for forming a catalyst layer of a PEM fuel cell;
(d) coating the slurry on one side of an electrode backing layer;
(e) drying the resultant material obtained in step (d) at a temperature less than or equal to a boiling point of the organic polar solvent, impregnating in an acid solution, washing and drying the resultant to form a gas diffusion electrode having a catalyst layer deposited on the electrode backing layer; and
(f) interposing a PEM between an anode side and a cathode side of the gas diffusion electrode to then hot-press the same.
To achieve the third objective, there is proviede a PEM fuel cell fabricated by the above method.
The principles of the present invention lie in modifying conventionally used perfluorocarbon polymer by adding an organic solvent thereto, thereby solving a problem arising due to alcohol remaining on the polymer, improving processing performance by facilitating preparation of a high viscosity slurry and improving power characteristics of the PEM fuel cell, in fabricating anodic and cathodic electrochemical catalyst layers of the PEM fuel cell. Also, according to the present invention, before forming a MEA structure, the M+ form-PFSI in the catalyst layer is protonated in a gas diffusion electrode state in which the catalyst layer is deposited on the backing layer, thereby shortening the overall processing time due to a reduced protonation time, compared to the conventional thin film forming method.